Stem cells represent unique cell populations that have the ability to undergo both self-renewal and differentiation. Although "adult" stem cells may develop into only a limited number of cell types, in contrast, early repopulating stem cells have the ability to initiate lineage-specific differentiation programs and to form numerous cell types in vitro. Although several studies have demonstrated that transcription factors play a significant role in early stem cell differentiation, extracellular hematopoietic cytokine regulatory signals appear to be more influential. Cytokine production has been extensively investigated using conventional ELISA and bioassay techniques, however, these methods typically measure cytokine secretion in cell culture supernatants, making characterization of secretory profiles of single cells difficult. Using single cell gel microdrop (GMD) encapsulation technology and flow cytometry, this SBIR aims to develop a multi- parameter single cell assay to detect cell surface marker expression and cytokine secretion simultaneously on single, viable hematopoietic stem/progenitor cells. The proposed single cell assay will provide a method for assessing the secretory profile of lineage specific stem cells and help to increase our understanding of stem cell differentiation for research and clinical applications including development of cellular therapies. [unreadable] [unreadable] [unreadable]